Balancing devices of this nature are already known for the purpose of compensating the couples exerted by gravity forces on a robot arm which is hinged to rotate about a horizontal axis. In this way, the energy of the arm drive members is used solely for driving the arm.
Prior art balancing devices suffer from a major drawback of being approximate and of being incapable of exactly compensating couples due to the force of gravity, particularly since these couples vary as a function of the angular position of the robot arm relative to the vertical.
A device for balancing gravity forces and serving to mitigate this drawback is described in French patent No. 84 08383, published under the No. 2 565 153 to which U.S. Pat. No. 4,620,829 corresponds.
The balancing device of the above-mentioned patent comprises a rigid auxiliary part hinged about a second axis parallel to the axis of the robot arm, said part comprising two transverse arms extending on either side of the second axis and having ends which are symmetrically disposed about said axis, said ends being suitable for bearing against a rectilinear portion of a member which moves in translation and which is subjected to a return force towards an initial position, said return force being proportional to the distance between said initial position and the position occupied by the moving member, together with means for rotating said auxiliary part about the second axis at an angular speed whose absolute value is equal to one half of the angular speed of the robot arm.
In this prior balancing device, the assembly is set up angularly in such a manner that when the robot arm is vertical, the two ends of the transverse arms bear simultaneously against said rectilinear part which is then occupying its initial position so that the moving member is not subjected to any return force, with the constant of proportionality between the return force and the amplitude of the displacement of the moving member, and with the distance between the second axis and the ends of the transverse arms both being chosen in such a manner as to ensure balance.
Although this prior device balances the arm properly regardless of its angular position, it nevertheless suffers from certain drawbacks.
Firstly, the device requires accurate guidance to ensure that the moving member moves in translation.
Further, outside the above-mentioned initial position, the bearing force exerted by the auxiliary part on the rectilinear portion of the moving member is offset relative to the translation axis of the moving member for all positions thereof. This means that the device runs the risk of jamming, particularly since the moving member is subjected to a return force which is high. Further, an increase in the forces to be balanced may give rise to an increase in the length of the transverse arms of the auxiliary part, which has the consequence of further increasing the offset effect and thus increasing the risk of jamming for the member which moves in translation. As a result, the very structure of the prior balancing device limits the maximum value of couple the device can balance.
In addition, in order to exert the return force, this prior device requires members such as compression springs which increase in size with increasing return force to be exerted.
It should also be mentioned that this prior device is particularly bulky and difficult to house in an installation including a robot arm, where available volume is often limited.
Finally, the prior balancing device requires the robot arm to be modified in order to be adapted thereto.
Consequently, one of the objects of the invention is to provide a balancing device of the type described in the above-mentioned French patent, but enabling the drawbacks of the prior art to be avoided.
A particular object of the invention is to provide such a balancing device which does not require accurate guidance means for the member which moves in translation and which does not run the risk of jamming.
Another object of the invention is to provide such a balancing device of reduced bulk which is also easily adapted to a robot arm without it being necessary to modify or transform the robot arm.
Yet another object of the invention is to provide such a balancing device capable of balancing couples of high value while the device remains small in size and while reducing the internal stresses specific to the device as much as possible.